The present invention relates in general to glazed nuclear fuel pellets and glazing compositions therefor, and more particularly, to the use of cadmium-113 isotope in the form of cadmium oxide (CdO) as a burnable absorber, also referred to as a burnable poison, in a glazing composition including other glass forming oxides for glazing nuclear fuel pellets for controlling the nuclear reactivity of a reactor core and ultimately extending the operating life cycle of the nuclear reactor.
The process of nuclear fission involves the disintegration of fissionable nuclear fuel material into two or more fission products of lower mass number. Among other things, the process also includes a net increase in the number of available free neutrons which are the basis for a self-sustaining reaction. When a reactor has operated over a period of time, the fuel assembly with fissionable materials must ultimately be replaced due to depletion. Inasmuch as the process of replacement is time consuming, taking as much as six weeks, and costly in terms of lost power generation, it is desirable to extend the life of a given fuel assembly as long as practically feasible. For that reason, deliberate additions to the reactor fuel of parasitic neutron-capturing elements in calculated small amounts may lead to highly beneficial effects on a thermal reactor. Such neutron-capturing elements are usually designated as burnable absorbers if they have a high probability or cross section for absorbing neutrons while producing no new or additional neutrons or changing into new absorbers as a result of neutron absorption. During reactor operation, the burnable absorbers are progressively reduced in amount so that there is a compensation made with respect to the concomitant reduction in the fissionable material.
The life of a fuel assembly may be extended by combining an initially larger amount of fissionable material, as well as a calculated amount of burnable absorber. During the early stages of operation of such a fuel assembly, excessive neutrons are absorbed by the burnable absorber which undergoes transformation to elements of low neutron cross section which do not substantially affect the reactivity of the fuel assembly in the latter period of its life when the availability of fissionable material is lower. The burnable absorber compensates for the larger amount of fissionable material during the early life of the fuel assembly, but progressively less absorber captures neutrons during the latter life of the fuel assembly, so that a long life at relatively constant fission level is assured for the fuel assembly. Accordingly, with a fuel assembly containing both fissionable material and burnable absorber in carefully proportioned quantities, an extended fuel assembly life can be achieved with relatively constant neutron production and reactivity. Burnable absorbers which may be used include boron, gadolinium, cadmium, samarium, europium, and the like, which upon the absorption of neutrons result in isotopes of sufficiently low neutron capture cross section so as to be substantially transparent to neutrons.
The incorporation of burnable absorbers in fuel assemblies has thus been recognized in the nuclear field as an effective means of increasing fissionable material capacity and therby extending reactor core life, for example, to eighteen months without the requirement for fissionable material replacement. Burnable absorbers are used either uniformly mixed with the fissionable material, i.e., distributed absorber, deposited as a coating on the exterior of nuclear fuel pellets containing fissionable material as disclosed in U.S. Pat. No. 3,427,222, or are placed discretely as separate elements in the reactor core. Thus, the net reactivity of the reactor core can be maintained relatively constant over the active life of a reactor core.
Where burnable absorbers are deposited as a coating on the exterior of nuclear fuel pellets, boron containing compounds such as boron carbide (B.sub.4 C), boron nitride (BN) and zirconium diboride (ZrB.sub.2) are most frequently used. Boron containing burnable absorbers may be applied as a coating of predetermined thickness to nuclear fuel pellets by a variety of techniques, for example, dip coating a nuclear fuel pellet in a composition containing a boron compound and a ceramic binder as disclosed in the above-mentioned United States Patent. However, the use of boron containing compounds as a burnable absorber is known to have a number of undesirable characteristics. For example, boron has a moderate burnout rate which often leaves residual burnable absorber within the coating at the end of any given time, thereby often adversely affecting the calculated control of the nuclear reactivity over the operating life cycle of the nuclear reactor. In addition, the burnout of boron from the coating often results in the retention within the fuel rods of undesirable gases produced by the boron burnout, thereby also adversely affecting the performance of the nuclear fuel rod and burnable absorber.
Accordingly, it can be appreciated that there is an unsolved need for a glazed nuclear fuel pellet and a glazing composition therefor which includes one or more burnable absorbers having a controlled increased rate of burnout without producing undesirable gases such that the nuclear reactivity of a reactor core can be effectively controlled and ultimately extending the operating life cycle of the nuclear reactor.